Skin acts as a natural barrier between internal and external environments and therefore plays an important role in vital biological functions such as protection against mechanical and chemical injury, microorganisms, and ultraviolet damage. The health and appearance of skin, however, can deteriorate due to environmental factors, genetic makeup, nutrition, and sun exposure. Consumers desire to slow the gaining of skin and reduce the effects of aging, especially in the face and around the eyes. Radiant and clear skin appears youthful and is a sign of good health and vitality.
With aging, the outer skin layer (epidermis) thins, even though the number of cell layers remains unchanged. The number of pigment-containing cells (melanocytes), however, decreases. Therefore, the skin appears pale and translucent. Large pigmented spots (age spots, liver spots, or lentigos) may appear in sun-exposed areas. Changes in the connective tissue reduce the skin's strength and elasticity. This is known as elastosis. It is more noticeable in sun-exposed areas (solar elastosis). Elastosis produces the leathery, weather-beaten appearance common to farmers, sailors, and others who spend a large amount of time outdoors. Dehydration increases the risk of skin injury. Poor nutrition can also negatively influence the skin, causing dryness, rash, and puffiness.
Skin is broadly described as having two major components, the dermis and the epidermis. The dermis provides the epidermis with a solid support that nourishes the epidermis. The dermis is formed mainly by fibroblasts and an extracellular matrix, which includes primarily collagen, elastin and a substance known as ground substance. The epidermis is a desquamating pluristratified epithelium that is about 100 μm thick. The outermost part of the epidermis is a cornified layer (or stratum corneum) made from keratinocytes at the terminal stage of their differentiation (known as corneocytes). The cornified layer is responsible for the barrier function of the epidermis and provides the first line of defense from external insult. It helps protect skin from damage from physical, chemical, and infectious injuries. The cornified layer also helps protect the body from water loss.
Due to the structure and function of the skin, it presents unique challenges to the development of cosmetic and/or pharmaceutical products. Products for treating skin include active compounds, but many active compounds present challenges, for example, challenges with respect to solubility, stability, and/or effectiveness. It is difficult to stabilize certain active agents so they remain active during the shelf life of a product. It is also difficult to ensure that certain active agents penetrate the outer cornified layer of the skin to provide their desired effect.